1. Field of the Invention
The present invention relates to a novel iridium complex and a light-emitting element that uses the novel iridium complex. More particularly, the present invention relates to a novel iridium complex and a light-emitting element that uses the novel iridium complex that can be appropriately used in display elements, displays, backlights, electrophotography, illumination light sources, recording light sources, exposure light sources, read light sources, signs, signboards, interior optical communication and the like.
2. Description of the Related Art
At present, research and development relating to various kinds of display elements are actively being carried out. In particular, organic electroluminescent (EL) elements have received attention as promising display elements because EL elements can emit highly luminous light at a low voltage. For example, a light-emitting element having an organic thin film formed by vapor deposition of an organic compound has been known (Applied Physics Letters, Vol. 51, p. 913 (1987)). Light-emitting elements disclosed in the literature use tris(8-hydroxyquinolinate) aluminum complex (Alq) as an electron transporting material, and have a structure comprising a layer containing the electron transporting material and a layer containing a hole transporting material (an amine compound) laminated together. In comparison to conventional light-emitting elements comprising a single layer, light-emitting characteristics can be greatly improved with light-emitting elements comprising laminated layers. In recent years, the application of organic EL elements to color displays and white light sources has been actively investigated. In order to apply organic EL elements to such ends, it is necessary to improve the light-emitting properties of the organic EL elements, such as luminance and light-emitting lifespan, with regard to light-emitting elements that are capable of emitting light in blue, green and red colors, respectively.
As a light-emitting element having improved light-emitting properties, a light-emitting element utilizing light emission from an orthometalated iridium complex (Ir(ppy)3: tris-orthometalated complex of iridium (III) with 2-phenylpyridine) has been reported (Applied Physics Letters, Vol. 75, p. 4 (1999)). While it has conventionally been said that the external quantum efficiency of light-emitting elements is limited to 5%, light-emitting elements disclosed in the literature reach an external quantum efficiency of 8%, which exceeds the conventional limitation. However, the emitted light obtained from the light-emitting element is limited to green light emission, and the applicable range as a display is narrow. The applicable range as a display could be expanded if light-emitting elements having improved light-emitting characteristics for other colors could be provided. Thus, there has been a demand to improve light-emitting characteristics of light-emitting elements of other colors.
With respect to blue light-emitting elements, various light-emitting elements have been reported that use a distyrylallylene derivative, represented by DPVBi (4,4′-bis(2,2′-diphenylvinyl)biphenyl), and an analog thereof. However, there have been no reports of blue light-emitting elements having an external quantum efficiency exceeding 5%. If a blue light-emitting element having an external quantum efficiency exceeding 5% could be provided, it would be possible for a highly efficient organic EL element to display multicolors and white color, whereby application of light-emitting elements would be greatly advanced. Moreover, it would be possible to greatly reduce electrical power consumption and to realize large area display and long term use when the light-emitting element is applied to a display element.